1. Field of the Invention
The present invention relates generally to a reversible recording medium, and more particularly to a reversible thermosensitive recording medium capable of repeatedly forming and erasing images by utilizing a reversible change in the transparency or color of a thermosensitive layer therein by a temperature change.
2. Background of the Invention
A variety of information recording media have been developed to comply with the demands created by the expanding volume of information. A reversible thermosensitive recording medium has been attracting much attention recently for its capability of forming and erasing images repeatedly on demand, and at relatively low cost without complicated developing steps.
Reversible thermal recording media are known to be capable of carrying out forming and erasing images by reversibly changing optical properties between two states, a white opaque state (image formed state) and a transparent state (erased state), depending on temperature.
For example, recording materials which comprise low molecular weight organic materials such as higher fatty acids dispersed in matrix resins such as vinyl chloride-vinyl acetate copolymers and have relatively low glass transition temperatures, ranging from approximately 50.degree. C. to equal or less than 80.degree. C. (Japanese Laid-Open Patent Applications No. 54-119377 and 55-154198).
However, these recording materials remain transparent in temperature ranges as narrow as from 2.degree. to 4.degree. C., which is unsatisfactory for image recordings using reversible recording materials.
In view of the above problem, the present inventors proposed recording materials in Japanese Laid-Open Patent Application Nos. 63-39378, 2-1363, 3-2089, and 5-294066, which broaden the above-mentioned temperature range, by using different kinds of low molecular weight organic materials or a combination thereof as the recording materials. While it is not possible with this technique to completely erase images with a thermal print head, the images can be erased by heating for a relatively long heating period with a heating roller or a hot plate.
To improve the image erasure characteristics, the present inventors further proposed a recording material in Japanese Laid-Open Patent Application No. 7-820491, in which rates of change in: (1) heating temperature for the initiation of transparentization; (2) transparency; and/or (3) film thickness, are specified for the recording material. By use of this recording material, the erasure characteristics are improved.
Japanese Patent Publication No. 2597268 discloses a low molecular weight organic material in a thermosensitive recording medium, including a mixture of at least one long chain alkyl compound having a melting point of from 50.degree. to 100.degree. C. having at least one saturated fatty acid bisamide. It has a melting point of 110.degree. C. or higher with a mixing ratio of from 98:2 to 80:20, to broaden the range of the "transparentization" temperature and to improve the erasure characteristics with a thermal print head. However, the recording medium is difficult to form in an even layer because of difficulties caused by: (1) the relatively poor solubility of the bisamides in solvent, such as tetrahydrofuran or the like; and (2) an insufficient dispersion of the particles of this organic material into a matrix resin. The above-mentioned recording medium was found quite difficult to change into the white opaque state (i.e., image formed state) with a resulting poor image contrast, thereby being unsuited for practical use.
A reversible recording medium is generally used and/or stored under various ambient conditions. When the medium is used especially in a rewritable recording card such as, for example, a point-of-sales card, the conditions of card storage and handling by each individual can be quite varied. Therefore, it is important that the recording material used in a reversible recording medium have durability under diversified storage conditions.
In addition, once a recording medium is stored at its recorded opaque white state (at erased transparent state) for a period of time as long as, for example, several months, a change in recording or erasure characteristics results. Namely, when a recording or erasure is made with a thermal printhead or a hot stamp after a prolonged storage, a decrease in image contrast is found in recorded images (i.e., in the reflective optical density of the images at the recorded white opaque state), or in erased images (i.e., in the reflective optical density at the erased transparent state). The display quality of images of the recording medium may thus be deteriorated as a result of the prolonged storage.